Traditional AC to DC power supplies typically use a full wave bridge feeding a storage capacitor. The capacitor is fully charged twice per power line cycle at the peak of the full wave rectified sine wave, wherein a heavy pulse is drawn from the power line just before each peak, and little or no current at other times. A sufficiently sized capacitor maintains the load voltage relatively constant to the level required by the load, and the load is powered from the stored energy between the power line peaks. Electronic devices such as televisions, VCRs, and computers, typically employ a full wave bridge to make a DC voltage that is used as the input to one or more switching power supplies. The switching power supplies then provide the regulated DC voltages used by the rest of the system.
Advances in power semiconductors and the availability of inexpensive off the shelf control chips have increased the popularity of switching power supplies and expanded the usage as well as producing more sophisticated power schemes that handle higher power levels. However, this expanded usage and popularity has also created certain problems.
Power plant generators produce sine wave voltages and these generators function most efficiently when the load currents are also sine waves. Among other things, it allows for a constant torque on the generator shaft and minimizes mechanical stresses. The use of full wave bridges that draw power via a short spike twice per line cycle tends to cause an imbalance in the generation scheme. In addition, the transformers and other power transmission equipment only handle current up to some maximum peak level and having these full bridge circuits drawing power during short periods increases the peak currents at the same power level. Furthermore, the deviation caused by the full bridge operation generates significant harmonic content at high frequencies that can interfere with radio communication and other electronic equipment.
In view of these problems, the industry and governments have made efforts to minimize the effects from the periodic power draws for the full bridge operations. For example, regulations have already been enacted in the European Union that constrains the range that the load current may deviate from a pure sine in phase with the voltage for some types of loads. These regulations will likely get tighter in the future, be applied to smaller loads, and spread to other regions.
Certain technological solutions have been attempted with limited success. One approach is to insert a passive filter between the power line and the equipment drawing power in short spikes. The power line is then presented with a ‘smoother’ load current. These passive filters can be effective at reducing the radio frequency harmonics, but require prohibitively heavy, large, and expensive inductors to make the load appear sufficiently close to sinusoidal.
One scheme to address the problems caused by the periodic power draw is through power factor correction (PFC). PFC uses an active electronic technique to present a sinusoidal load to the power line regardless of the characteristics of the final load. There are a number of PFC topologies for achieving this. A common type is a type of switching power supply called a boost converter. The boost converter performs PFC by taking the raw rectified AC line as input and switches at many times the power line frequency such that the power line voltage changes relatively little between each boost pulse. The boost converter produces a voltage somewhat higher than the highest peak of the AC input line. For each boost pulse, the average current drawn from the AC line for that pulse interval is proportional to the instantaneous AC line voltage. The current drawn from the AC line is therefore sinusoidal and in phase with the voltage. Ideally, the load on the AC line appears resistive.
Although the current drawn from the AC line is dictated by the AC line voltage, the boost switcher output voltage is still controlled, but much more slowly than at each switching pulse. In other words, the resistance of the resistive load presented to the AC line is slowly varied according to the demands of the final load. The line current is still mostly proportional to the line voltage, but this proportionality “constant” is slowly varied over a number of line cycles.
The output of the boost switcher is therefore a DC voltage a bit higher than the AC line peak voltage with significant ripple at twice the line frequency.
Thus, the purpose of a PFC power supply is to draw power from an AC voltage (usually the power line) in such a way that the instantaneous current drawn is proportional to the instantaneous voltage. The current waveform therefore has the same shape and phase as the voltage waveform. Another way of stating this is that the load looks resistive to the AC line. This is desirable for many reasons well known in the art.
FIG. 1 shows one type of prior art PFC power supply known in the art. The raw AC input voltage is first full-wave rectified, and inductor 2, switch 3, and diode 4 form a boost type switching power supply. When switch 3 is closed, the absolute value of the AC input voltage is applied directly across inductor 2. The inductor current then rises linearly at a rate proportional to the absolute value of the AC input voltage at that time. This stores energy in inductor 2. When switch 3 is opened, inductor 2 will produce whatever voltage is required to force the same current to flow thru it until it is discharged. The only path this current can take is thru diode 4. Capacitor 5 charges to a higher voltage than the AC line input level. The current thru inductor 2 therefore decreases at a rate proportional to the voltage on capacitor 5 minus the absolute value of the AC input voltage at that time. It is important to note that the total time for inductor 2 to charge and discharge is very small compared to one AC line cycle, and the AC line voltage can therefore be considered constant during one charge/discharge cycle, or “switching pulse”.
The net result of the boost switching power supply formed by inductor 2, switch 3, diode 4, and capacitor 5 is that the voltage on capacitor 5 is higher than any point on the AC power line waveform. If this were not the case, then capacitor 5 would be charged directly thru inductor 2 and diode 4 until it is at the maximum of the AC power line. Any switching pulses would then raise it even further.
The switching element is shown as a simple switch 3 in the diagram. For the purpose of this discussion, it acts like a switch under control of the control element 6. In most cases switch 3 would be implemented as a transistor.
Note that the current drawn from the AC line is a function of how often and how long switch 3 is closed. In a PFC supply, these parameters are deliberately controlled so that the current drawn from the AC line is proportional to its voltage averaged over each pulse interval. To achieve this, the control element 6 requires the absolute value of the AC line input voltage 8 and the current being drawn from the AC line 7 as inputs. In the traditional scheme, control 6 uses feedback to null the difference between the input voltage signal 8 and the input current signal 7. If the current is too low, then switch 3 is closed more often and/or for longer, and vice-versa if the current is too high.
To be a useful power supply, the output voltage must be regulated within some limits. This is the purpose of feedback path 9 into the control element. However, the control element must typically respond to this feedback slowly or the PFC function is defeated. In essence, the PFC function presents a restive load to the AC line, and feedback 9 is used to slowly adjust that resistance to maintain a roughly uniform output voltage. Since the control element 6 can only respond slowly (several AC input cycles) to the feedback 9, the output voltage will have “ripple” on it due to the input AC line cycles. A careful tradeoff is used in processing the feedback 9 to be as responsive as possible to output load changes while not responding to the ripple caused by the AC input waveform.
Control element 6 has traditionally been implemented with analog electronics, which are well suited to adjusting an output to null the error between feedback signals. Recent advances in digital microcontrollers have allowed for the same control scheme to be implemented in digital electronics, although existing digital implementations merely perform the same control operations previously performed by analog electronics.
What is needed, therefore, are techniques and devices for improving the PFC design. Such a PFC design should be simple and easily integrated into current manufacturing technologies and devices. The system should more fully exploit the digital processing capabilities to provide a more efficient and powerful topology.